“I called [Secretary of State] Mike Pompeo last night to let him know I was resigning from that. I can’t do it. I can’t stay," Mulvaney told CNBC in an interview.
Mulvaney said more officials from Trump's administration could soon quit, while others plan to stay on because they fear the president could "put something worse in" before leaving office on 20 January.
On Wednesday, Trump loyalists broke into the US Capitol building after the president delivered a fiery speech to tens of thousands of supporters near the White House.
During the remarks, Trump said he refused to concede and called on his supporters to keep fighting to prevent the election from being “stolen,” claiming widespread vote fraud, allegations that have been refuted by election officials.